Raster scan caption generators are useful in television receivers for displaying various types of information such as teletext data, time and channel settings, computer data and so on. In a typical generator individual alphanumeric characters or graphic symbols are represented by a dot matrix pattern stored in a read only memory (ROM). A character is generated for display by transferring a desired dot pattern from the ROM to a high speed buffer and sequentially shifting the character "dots" or "pixels" (picture elements) out of the buffer with a pixel or dot clock. The serial signal, thus formed, is applied to a kinescope in a timed relation to the vertical and horizontal sweep so as to display the dot matrix pattern at a desired location on the raster.
In a known form of on-screen display (OSD) character generator used in television receivers, an inductance-capacitance (LC) or resistance-capacitance (RC) oscillator is used for providing the clock signal which determines the timing of character elements or "pixels" provided by the character generator. The oscillator frequency (about 5 MHz) determines the width of the smallest element of displayed characters. The oscillator must be synchronized or "line-locked" with the horizontal scanning signal to avoid a ragged or noisy appearance of vertical edges of displayed characters. Typically, the oscillator is of the "start-stop" kind which is disabled during the presence of the horizontal synchronizing pulse and enabled at the termination of the synchronizing pulse.
Where the oscillator is included on the same integrated circuit with the OSD character generator, two pins of the integrated circuit must be "dedicated" to providing connections for discrete external frequency determining components (e.g., RL or RC elements). These external components of the oscillator present a potential source of radiation which may interfere with other signals in the receiver and so require filtering to prevent artifacts from appearing in displayed images. A further problem is that tolerance variations of the LC or RC oscillator components may require a factory adjustment for correct positioning of displayed characters. Also, changes in operating temperature and aging of circuit components can result in noticeable changes in the horizontal position of displayed characters. Further disadvantages are that at least one of the oscillator elements in the known system must be frequency adjustable and the external elements require printed circuit board space for mounting which adds to the overall cost of the OSD feature of the television receiver.
Television receivers are known which include a 4 MHz crystal oscillator as a signal source for a frequency synthesis type of tuner. Receivers are under consideration wherein on-screen display logic is to be included on the same integrated circuit as the frequency synthesis tuning system. In view of the foregoing, it is herein recognized that it would be advantageous to use the existing 4 MHz crystal oscillator as a clock source for the OSD character generator. Heretofore, this possibility has not been considered to be practical for various reasons. For example, the 4 MHz oscillator must be running continuously to satisfy the requirements of the tuning system. But even if it were possible some way to re-design the frequency synthesizer logic to work with a start-stop oscillator, a crystal oscillator can not be started quickly enough to meet the needs of the OSD generator because of the very high "Q" of the crystal. When a typical crystal oscillator is enabled the oscillations build up slowly in a period of time measured in milliseconds. This is simply too slow to meet the clock timing requirements of an OSD character generator.